Miniature cameras are becoming increasing common in mobile electronic devices such as smartphones. There is a constant drive to improve performance of such cameras, whilst ideally maintaining the same envelope. One feature augmentation that is now standard in such miniature cameras is autofocus. The incumbent actuator technology for such cameras is the voice coil motor (VCM). Many other technologies have been proposed over the last few years, with varying strengths and weaknesses and differing degrees of commercial success. The VCM technology has the key advantage of being simple, and therefore being straightforward to design. Whilst there are several disadvantages of VCM, such as high power, and low relative force, their use persists. One technology that has showed promise over the last few years is silicon micro-electro-mechanical systems (MEMS).
The MEMS technology is based around the philosophy of the electrostatic comb drive. The magnitude of actuation movement achievable with such a small-scale silicon device is less than that required to move the whole lens in such miniature cameras. The MEMS technology allows focusing of the camera between the notional infinity object distance and 10 cm focal distance, which is the typical specification for most devices. In addition, the cost of the MEMS actuator is almost entirely proportional to the surface area of silicon per device since MEMS devices are manufactured at the wafer level. As such, current MEMS devices mount a single lens element from the multi-element lens stack typically found in such miniature camera lenses. By choosing a lens element with high optical power, and hence a small relative focal length, the actuation movement is reduced. Since a single lens element is mounted in the actuator, the size of the silicon actuator can also be minimized.
As compared with VCM, the MEMS technology benefits from requiring very little power, a factor that is increasingly important in mobile devices. In addition, owing to the small size and stiff silicon structure, the mechanical resonance is much higher, delivering much faster response speed and focusing time, and also better stability for different camera orientations. There are two key factors, however, that have prevented the MEMS technology from being a practical commercial success. The first is that the actuator has thus far failed to survive the very difficult impact and drop testing that mobile devices require. In addition, moving a single lens element increases the complexity of the lens assembly and means that the actuator assembly and lens assembly are no longer separated, but integrated. This presents manufacturing problems.